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Home : 2002 : Feb : 12

    evaporation
    By Julianne

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    It's very hard for young students to visualize water vapor and the evaporation/condensation cycle. I use a cup of snow that melts, then slowly "disappears". We talk about where it could have gone - the janitor might have drunk it! Jesse might have knocked it over. Finally someone usually suggests that it went into the air. I ask them how we could test that theory. I lead them to the idea that we could have an uncovered cup and a covered cup and see if they both disappear. Of course, the uncovered cup evaporates and the covered cup stays the same. Then we talk about what caused the change. What caused the snow to melt? Would it melt outside? Why or why not? Once they come up with the idea of heat causing the change we make the whole process happen very quickly by melting an ice cube over a flame. The kids can actually see the water turn into water vapor which helps them better understand evaporation.

    After doing the snow to water to vapor experiment I ask them to think of a way to reverse the process. Of course they think of refreezing the water to make ice, but how do you condense water vapor into water? The easiest way I've found is to fill a glass jar with cold water and ice. Seal it securely and wipe it dry. Let it stand on a paper towel until you see water condensing on the outside of the jar. How did it get there? If they think it came from inside the jar, redo the experiment using room temperature water in the jar. This time it won't "sweat". So the water on the outside of the jar must have come from the air.



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